Studies of the DLR-Institute of Atmospheric Physics with a global climate model published in Atmospheric Chemistry and Physics recently show a high impact of gaseous and particulate emissions from ocean-going ships on maritime clouds. The additional aerosol particles brighten the clouds above the oceans, which then are able to reflect more sunlight back into space. Although the uncertainties associated with this study are still high, the model results clearly indicate that the cooling due to altered clouds far outweighs the warming effects from greenhouse gases such as carbon dioxide (CO2) or ozone from shipping, overall causing a negative radiative forcing today. The indirect aerosol effect of ships on climate is found to be far larger than previously estimated contributing up to 39% to the total indirect effect of anthropogenic aerosols. This contribution is high because ship emissions are released in regions with frequent low marine clouds in an otherwise clean environment and the potential impact of particulate matter on the radiation budget is larger over the dark ocean surface than over polluted regions over land. The main reason for the high impact on clouds is the high average sulphur content in maritime fuels.

A large fraction of ship emissions occurs within 400 km of coastlines, causing air quality problems in coastal areas and harbours with heavy traffic. Pollution from marine shipping causes numerous premature cardiopulmonary and lung cancer deaths around the world each year, according to a report led by scientists of the University of Delaware and Rochester Institute of Technology (USA) published recently in Environmental Science and Technology, with impacts concentrated in coastal regions along major trade routes. CO2 and sulphur dioxide (SO2) from ships also contribute to acidification of the ocean, which poses a potential threat to marine biota. In addition, CO2 remains in the atmosphere for a long period of time and will therefore continue to have a warming effect long after its emission. In contrast, sulphate has an atmospheric residence time of typically only a few days, and the climate response from sulphate is of the order decades while that of CO2 is on the order of centuries. Thus, shipping with sulphur rich fuel should not be considered as an option to face global warming caused by continuous emissions of greenhouse gases.

We conclude that while the control of NOx, SO2 and particle emissions from ships will have beneficial impacts on air quality and acidification, CO2 reductions from all sources, including ships and other freight modes, are urgently required to reduce global warming.

The research was supported by the German Helmholtz-Gemeinschaft Deutscher Forschungszentren and the German Aerospace Center (DLR) within the Young Investigators Group SeaKLIM and by the Oak Foundation.